Electrically conductive porous particle

ABSTRACT

There is provided a method of forming a porous particle comprising an electrically conductive continuous shell encapsulating a core, said core comprising an elemental compound that reversibly reduces in the presence of a cation and oxidizes in the absence of said cation, said method comprising the steps of: a) encapsulating an elemental compound precursor with said electrically conductive shell; b) reacting said elemental compound precursor with an oxidation agent to oxidise said elemental compound precursor to form said elemental compound, thereby forming said electrically conductive shell encapsulating said core comprising said elemental compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 15/311,184, filed Nov. 14, 2016, which is a U.S.National Phase Application under 35 U.S.C. § 371 of InternationalApplication No. PCT/SG2015/050114, filed May 14, 2015, entitled A METHODOF FORMING A POROUS PARTICLE, which claims priority to Singapore PatentApplication No. 10201402315P, filed May 14, 2014.

TECHNICAL FIELD

The present invention generally relates to a method of forming a porousparticle and uses of the porous particle.

BACKGROUND ART

High-performance batteries can serve as part of a solution to supply andstorage problems as well as environmental issues related to thereplacement of fossil-fuel-based energy with clean alternative energy.Rechargeable batteries (also called secondary batteries) are anessential tool on the way to achieve this goal.

Rechargeable batteries differ from their non-rechargeable counterpartsin that they may be connected to an electricity supply, such as a wallsocket, be recharged and used again. In rechargeable batteries, eachcharge/discharge process is called a cycle. Rechargeable batterieseventually reach an end of their usable life, but typically only aftermany charge/discharge cycles.

Currently, rechargeable lithium-ion (Li-ion) batteries are the mostwidely used rechargeable batteries. Existing Li-ion batteries are basedon LiCoO₂ cathodes and graphite anodes. This leading Li-ion batterytechnology is based on intercalation reactions and is believed to belimited to a theoretical specific energy density of ˜370 Wh kg⁻¹ andtheoretical specific capacity <200 mAh g⁻¹ for the LiCoO₂/graphite.

Rechargeable lithium-sulfur batteries (Li—S), on the other hand, are ofinterest because of the high theoretical specific energy density (2600Wh kg⁻¹), high theoretical specific capacity (1680 mAh g⁻¹), lowmaterial cost, and low safety risk that they possess.

However, lithium-sulfur batteries suffer from a number of drawbacks suchas (1) leakage of sulfur and polysulfides from the host during dischargeprocess; 2) poor control over the free volume that shall be in placeinside of the cathode materials to accommodate the large volumeexpansion of sulfur by lithiation during discharge process (formation ofLi₂S, corresponding to ˜78.7% of volume expansion); and 3) poorelectrical conductivity of the bulk sulfur. All these factors will leadto capacity fading and poor cyclability of the lithium-sulfur batteries.These disadvantages cannot be resolved by the current available methodsto form such rechargeable lithium-sulfur batteries.

There is therefore a need to provide a method of forming a component ofa rechargeable battery that overcomes, or at least ameliorates, one ormore of the disadvantages described above.

SUMMARY OF INVENTION

According to a first aspect, there is provided a method of forming aporous particle comprising an electrically conductive shellencapsulating a core, said core comprising an elemental compound thatreversibly reduces in the presence of a cation and oxidizes in theabsence of said cation, said method comprising the steps of:

a) encapsulating an elemental compound precursor with said electricallyconductive shell;

b) reacting said elemental compound precursor with an oxidation agent tooxidise said elemental compound precursor to form said elementalcompound, thereby forming said electrically conductive shellencapsulating said core comprising said elemental compound.

Advantageously, this method may allow for the encapsulation of anelemental compound that would normally be destroyed by currentlyavailable methods of forming a lithium-sulfur battery.

The encapsulation of the elemental compound may also ensure that theelemental compound is not lost during discharge of the battery as theelemental compound is confined by the electrically conductive shell.

Further advantageously, this method may allow for the core to possess asmaller volume than the inner volume of the encapsulating shelltherefore creating a void inside the shell, in which the void isdisposed between the core and the shell. Due to the presence of thevoid, the core may increase in volume without damaging or destroying theencapsulating shell. The void may be formed during the reacting step(b).

The method may result in a plurality of porous particles being formed inwhich the electrically conductive shells are in close proximity witheach other. Due to the interconnected shells, this may ensure that goodelectrical contact is maintained from one particle to the other,ensuring that the plurality of porous particles (which may form amatrix) has good electrical conductivity as a whole. The encapsulatedelemental compound within each porous particle then encounters lesserelectrical resistance as compared to a bulk elemental compound (that is,one that is not encapsulated by an electrically conductive shell).

According to a second aspect, there is provided a porous particlecomprising an electrically conductive shell encapsulating a core, saidcore comprising an elemental compound precursor that oxidizes in thepresence of an oxidation agent to form an elemental compound.

According to a third aspect, there is provided a cathode comprising aplurality of porous particles, each porous particle made according tothe method as defined above, wherein each porous particle comprises anelectrically conductive shell encapsulating a core, said core comprisingan elemental compound that reversibly reduces in the presence of acation and oxidizes in the absence of said cation.

Advantageously, choosing the elemental compound so that it mayreversibly be reduced in the presence of a cation and oxidized in theabsence of that cation may allow the porous particle to be used in abattery such as a rechargeable battery.

According to a fourth aspect, there is provided a battery comprising ananode; the cathode as defined above; and an electrolyte comprising thecation.

Advantageously, having a void inside the shell may allow the core toincrease in volume without damaging or destroying the shell. This mayaid in minimizing loss in efficiency when the battery charges anddischarges repeatedly.

Definitions

The following words and terms used herein shall have the meaningindicated:

The term ‘porous particle’ is to be interpreted broadly to refer to aparticle having a structure containing pores. The pores may be presenton the outer surface of the particle or may extend from the outersurface to a point in the inner volume of the particle. Where theparticle has a core-shell configuration, the pores may be present in theshell. The pores may be present on the outer surface of the shell or mayextend throughout a part of or the entire thickness of the shell. Theporous structure may allow the movement of chemicals between an externalenvironment and the interior of the particle.

The term ‘electrically conductive’ is to be interpreted broadly to referto a material or compound having a measurable level of electricalconductivity and that which allows an electric current to be passedthrough.

The term ‘void’ is to be interpreted broadly to refer to a space orvolume where no solid matter is present, but in which gas and/or aliquid can be present.

The term ‘nanoparticle’ is to be interpreted broadly to refer to aparticle possessing a dimension that less than about 1000 nm, less thanabout 500 nm or less than about 100 nm.

The term ‘elemental compound’ is to be interpreted broadly to refer to acompound that is formed by one or several identical atoms such as O₂,S₈, N₂, Fe, I₂ etc. The phrase ‘elemental compound that reversiblyreduces in the presence of a cation and oxidizes in the absence of saidcation’ is to be interpreted broadly to refer to a sequence of chemicalreactions where firstly an elemental compound is being reduced by way ofa spontaneous chemical reduction reaction when in contact with thecation to form a complex and secondly reversion of the complex to theelemental compound and the cation (whether as separate molecules or aswhen present as an intermediary complex as long as there is an increasein the oxidation state of the elemental compound) by way of a chemicaloxidation reaction when the cation dissociates from the complex.

The word “substantially” does not exclude “completely” e.g. acomposition which is “substantially free” from Y may be completely freefrom Y. Where necessary, the word “substantially” may be omitted fromthe definition of the invention.

Unless specified otherwise, the terms “comprising” and “comprise”, andgrammatical variants thereof, are intended to represent “open” or“inclusive” language such that they include recited elements but alsopermit inclusion of additional, unrecited elements.

As used herein, the term “about”, in the context of concentrations ofcomponents of the formulations, typically means +/−5% of the statedvalue, more typically +/−4% of the stated value, more typically +/−3% ofthe stated value, more typically, +/−2% of the stated value, even moretypically +/−1% of the stated value, and even more typically +/−0.5% ofthe stated value.

Throughout this disclosure, certain embodiments may be disclosed in arange format. It should be understood that the description in rangeformat is merely for convenience and brevity and should not be construedas an inflexible limitation on the scope of the disclosed ranges.Accordingly, the description of a range should be considered to havespecifically disclosed all the possible sub-ranges as well as individualnumerical values within that range. For example, description of a rangesuch as from 1 to 6 should be considered to have specifically disclosedsub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4,from 2 to 6, from 3 to 6 etc., as well as individual numbers within thatrange, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of thebreadth of the range.

Certain embodiments may also be described broadly and genericallyherein. Each of the narrower species and subgeneric groupings fallingwithin the generic disclosure also form part of the disclosure. Thisincludes the generic description of the embodiments with a proviso ornegative limitation removing any subject matter from the genus,regardless of whether or not the excised material is specificallyrecited herein.

Detailed Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a method of forming a porousparticle comprising an electrically conductive continuous shellencapsulating a core, the core comprising an elemental compound thatreversibly reduces in the presence of a cation and oxidizes in theabsence of said cation, will now be disclosed.

The method may comprise the steps of (a) encapsulating an elementalcompound precursor with the electrically conductive shell; and (b)reacting the elemental compound precursor with an oxidation agent tooxidise the elemental compound precursor to form the elemental compound,thereby forming the electrically conductive shell encapsulating the corecomprising the elemental compound.

In the method, when the elemental compound precursor is oxidized in step(b), a void that is disposed between the shell and the core may becreated. The volume occupancy of the void between the shell and the coremay be about 10% to about 95%, about 10% to about 20%, about 10% toabout 30%, about 10% to about 40%, about 10% to about 50%, about 10% toabout 60%, about 10% to about 70%, about 10% to about 80%, about 10% toabout 90%, about 20% to about 95%, about 30% to about 95%, about 40% toabout 95%, about 50% to about 95%, about 60% to about 95%, about 70% toabout 95%, about 80% to about 95%, about 90% to about 95%, or about 40%to about 80%.

The elemental compound may be selected from the Group 15 or 16 of thePeriodic Table of Elements. Hence, the elemental compound may beselected from the group consisting of phosphorus, arsenic, antimony,sulfur, selenium, tellurium and polonium. The elemental compound may besulfur. The elemental compound may reduce by way of a spontaneouschemical reduction reaction when in contact with a cation to form acomplex, which reverts to the elemental compound and the cation (whetheras separate molecules or as when present as an intermediary complex aslong as there is an increase in the oxidation state of the elementalcompound) by way of a chemical oxidation reaction when the cationdissociates from the complex. The complex may have the formulaM_(x)EC_(n), where M refers to the cation, x refers to the number ofatoms of the cation, EC refers to the elemental compound and n refers tothe number of atoms of the elemental compound.

The elemental compound precursor may be a metal chalcogenide. The metalof the metal chalcogenide may be selected from Group 7, 8, 9, 10, 11,12, or 14 of the Periodic Table of Elements. The metal may be selectedfrom the group consisting of manganese, iron, cobalt, nickel, copper,zinc, lead, silver and cadmium. Hence, where the elemental compound issulfur, the metal chalcogenide may have the formula MS or M₂S. The metalchalcogenide may have one or more of the following properties (1) a highmelting point and a high decomposition temperature in a reducingatmosphere, (2) a low solubility product constant (K_(sp)) that allowsfor high-yield synthesis of the porous particle, (3) be environmentalfriendly, and/or (4) low cost and abundant in supply. The metalchalcogenide may be of uniform shape and/or size. The metal chalcogenidemay be a metal sulfide selected from, but not limited to, MnS, FeS, CoS,NiS, CuS, Cu₂S, ZnS, PbS, Ag₂S, or CdS. The metal sulfide may be ZnS.The various properties of the metal sulfides listed above are providedin Table 1 below.

TABLE 1 Max. temperature Sulfide K_(sp) Melting point stable in H₂ ² MnS3 × 10⁻¹¹ 1610 1244 FeS 8 × 10⁻¹⁹ 1118 273 CoS 5 × 10⁻²² 1117 134 NiS 4× 10⁻²⁰  976 129 CuS 8 × 10⁻³⁷    507¹ −39.7 ZnS 2 × 10⁻²⁵ 1700 979 PbS3 × 10⁻²⁸ 1114 423 Ag₂S 8 × 10⁻⁵¹  825 −76.6 CdS 1 × 10⁻²⁷ 1750 741¹Decomposed into Cu₂S ²Based on the thermal dynamic calculation ΔG = ΔH− TΔS

The elemental compound precursor may take on a spherical shape or atleast substantially spherical. The diameter of the elemental compoundprecursor (or equivalent diameter thereof) may be in the range of about50 nm to about 5000 nm, about 50 nm to about 100 nm, about 50 nm toabout 500 nm, about 50 nm to about 1000 nm, about 50 nm to about 2000nm, about 50 nm to about 3000 nm, about 50 nm to about 4000 nm, about100 nm to about 5000 nm, about 500 nm to about 5000 nm, about 1000 nm toabout 5000 nm, about 2000 nm to about 5000 nm, about 3000 nm to about5000 nm, or about 4000 nm to about 5000 nm.

The elemental compound precursor may be thermally stable.

As mentioned above, a void may be present between the shell and the coreas the elemental compound precursor oxidizes. The void may beattributable to the loss of the metal from the metal chalcogenide as themetal chalcogenide oxidizes to form the elemental compound (which is thechalcogen). This may be due to the differences in the density andformula weight of the elemental compound precursor and elementalcompound, leading to significant volume shrinkage within the core regionof the particle. The resultant particle may be deemed to have ayolk-shell structure in which the elemental compound forms the yolk ofthe electrically conductive shell. Where the porous particle is used ina rechargeable battery, the presence of the void within the porousparticle may allow for the expansion of the elemental compound as therechargeable battery discharges. The yolk-shell strategy then results inelemental compound protected inside a protective shell while having avolume much smaller than the electrical conductivity required forbattery application.

The void volume can be controlled or tuned by controlling the reactionconditions or amounts of chemicals used. For example, the concentrationof the oxidation agent, and/or the oxidation time can be controlled.

The oxidation agent may be selected from, but is not limited to, Fe³⁺,C³⁺, Sn⁴⁺, MnO₄ ⁻, Cr₂O₇ ²⁻, ClO₄ ⁻, ClO₃ ⁻, HNO₃, F₂, O₂, O₃, Cl₂, Br₂,I₂ and I₃ ⁻. The salts of Fe³⁺, Co³⁺ and Sn⁴⁺ ions may include, but arenot limited to, acetate, chloride, nitrate, sulfate and phosphate. Thesalts of MnO₄ ⁻, Cr₂O₇ ²⁻, ClO₄ ⁻ and ClO₃ ⁻ ions may include, but arenot limited to, lithium, sodium and potassium salts.

Where the metal chalcogenide is a metal sulfide, the elemental compoundis sulfur and the oxidation agent is a ferric salt, the metal sulfide isoxidised to elemental sulfur via the following reaction:[Math. 1]MS_((s))÷2 Fe³⁺ _((aq))→M²⁺ _((aq))+S_((s))+2Fe²⁺ _((aq))  (1)

The method may further comprise the step of forming the electricallyconductive shell. The electrically conductive shell may be a continuousshell in which the electrically conductive shell may substantially coator cover most if not all of the surface of the elemental compoundprecursor (as well as the resultant elemental compound). Theelectrically conductive shell may be porous. The electrically conductiveshell may contain micropores which allow the entrance of an oxidationagent into the core region of the particle and the egress of freed ions(generated from the oxidation of the elemental compound precursor) fromthe core region of the particle.

The electrically conductive shell may be carbon. The carbon may begraphite, graphene, carbon nanotubes or amorphous carbon (such as carbonblack). The carbon shell may be derived from a carbon precursor. Thecarbon precursor may be mixed with the elemental compound precursor toform a mixture. In the mixture, the carbon precursor may at leastpartially coat the elemental compound precursor. The carbon precursormay be in a resin form so as to hold or support the elemental compoundprecursor in a matrix configuration. The mixture may then be subjectedto a carbonization process in which the mixture is heated to atemperature in the range of about 200° C. to about 1000° C., about 200°C. to about 300° C., about 200° C. to about 400° C., about 200° C. toabout 500° C., about 200° C. to about 600° C., about 200° C. to about700° C., about 200° C. to about 800° C., about 200° C. to about 900° C.,about 300° C. to about 1000° C., about 400° C. to about 1000° C., about500° C. to about 1000° C., about 600° C. to about 1000° C., about 700°C. to about 1000° C., about 800° C. to about 1000° C., or about 900° C.to about 1000° C. The carbonization temperature may be about 900° C.

The carbon precursor may be an organic compound. The organic compoundmay be a polymer selected from the group consisting of a polyalkylene,polystyrene, polyacrylate, poly halide, polyester, polycarbonate,polyimide, phenol formaldehyde resin, epoxy, polyalkylene glycol andpolysaccharide. The polymer may be selected from the group consisting ofpolyethylene, polypropylene, polymethylmethacrylate, polyvinyl chloride,polyethylene terephthalate, polyethylene glycol, polypropylene glycol,starch, glycogen, cellulose and chitin.

Hence, the method may be viewed as an indirect pathway of preparing aporous particle in which an elemental compound is present in the core ofa porous particle. The thermally stable elemental compound precursor maybe coated with a carbon precursor and may undergo a carbonizationreaction to form the electrically conductive carbon shell on theelemental compound precursor. Hence, the elemental compound precursorcan be regarded as a sacrificial template that is able to withstand thehigh temperature required for the carbonization reaction. The elementalcompound precursor then undergoes an oxidation reaction in the presenceof an oxidation agent to form the elemental compound and free ions. Dueto the differences in density and formula weight of the elementalcompound precursor and the elemental compound, significant volumeshrinkage is observed during the oxidation process. Such shrinking leadsto the creation of the desired yolk-shell structure characterised by avoid between the electrically conductive shell and the elementalcompound. The freed ions then slowly leeched out from the intrinsicinterstitial void and micropores on the carbon shell without damagingthe carbon shell to form the resultant porous particle.

In this indirect pathway, direct carbonization of the elemental compoundis avoided. By avoiding direct carbonization (in which the elementalcompound sulfur will be subjected to a high temperature which can causethe vaporization and disappearance of the elemental compound), thismethod ensures that the elemental compound is retained in the porousparticle and be encapsulated by the electrically conductive shell.

The method may lead to a porous particle having different weight ratiosbetween the elemental compound and the shell. The weight ratio of theelemental compound to the shell may be in the range of about 0.5:1 toabout 3:1. The weight ratio of the elemental compound to the shell maybe about 0.5:1, 1:1 or 3:1. The weight percentage of the elementalcompound may be in the range of about 10% to about 90%, about 10% toabout 20%, about 10% to about 40%, about 10% to about 60%, about 10% toabout 80%, about 20% to about 90%, about 40% to about 90%, about 60% toabout 90%, about 80% to about 90%, or about 20% to about 60%. The amountof the elemental compound in the porous particle can be controlled bycontrolling the amount of carbon precursor used.

The volume occupancy of the core (made up of the elemental compound)within the porous particle may be about 5% to about 90%, about 5% toabout 10%, about 5% to about 20%, about 5% to about 30%, about 5% toabout 40%, about 5% to about 50%, about 5% to about 60%, about 5% toabout 70%, about 5% to about 80%, about 10% to about 90%, about 20% toabout 90%, about 30% to about 90%, about 40% to about 90%, about 50% toabout 90%, about 60% to about 90%, about 70% to about 90%, or about 80%to about 90%.

The porous particle may have a diameter in the range of about 50 toabout 5000 nm, about 50 to about 100 nm, about 50 to about 500 nm, about50 to about 1000 nm, about 50 to about 2000 nm, about 50 to about 3000nm, about 50 to about 4000 nm, about 100 to about 5000 nm, about 500 toabout 5000 nm, about 1000 to about 5000 nm, about 2000 to about 5000 nm,about 3000 to about 5000 nm, about 4000 to about 5000 nm, or about 100to about 500 nm.

The porous particle may have a shell thickness in the range of about 1to about 50 nm, about 1 to about 10 nm, about 1 to about 20 nm, about 1to about 30 nm, about 1 to about 40 nm, about 10 to about 50 nm, about20 to about 50 nm, about 30 to about 50 nm, about 40 to about 50 nm, orabout 5 to about 10 nm.

The method may result in a plurality of porous particles being formed inwhich the electrically conductive shells are in close proximity witheach other. Due to the interconnected shells, this may ensure that goodelectrical contact is maintained from one particle to the other,ensuring that the plurality of porous particles (which may form amatrix) has good electrical conductivity as a whole. Hence, theplurality of porous particles may form an interconnected 3-dimensionalmatrix where the electrically conductive shells are in close proximityor in direct contact with each other.

There is also provided a porous particle comprising an electricallyconductive shell encapsulating a core, the core comprising an elementalcompound precursor oxidizes in the presence of an oxidation agent toform an elemental compound. The elemental compound may reversibly reducein the presence of a cation and oxidizes in the absence of the cation.As the elemental compound is formed during oxidation of the elementalcompound precursor, a void is formed which is present between the shelland the elemental compound.

There is also provided a cathode comprising a plurality of porousparticles, each porous particle made according to the method as definedherein, wherein each porous particle comprises an electricallyconductive shell encapsulating a core, the core comprising an elementalcompound that reversibly reduces in the presence of a cation andoxidizes in the absence of the cation.

There is also provided a battery comprising (a) an anode; (b) thecathode as defined herein; and (c) an electrolyte comprising the cation.In the battery, the cation may be dissociated from the anode. The cationmay be chosen from the group consisting of lithium, sodium, potassium,rubidium, beryllium, magnesium, calcium, strontium and barium.

The anode may be chosen from, but not limited to, lithium, sodium,potassium, rubidium, beryllium, magnesium, calcium, strontium, barium ora graphene rod plated with any of these metals or a combination thereof.

The cathode electrode may be manufactured by, but not limited to, mixingthe porous particles in an organic solvent along with a binder and aconductive material to produce a slurry, applying the slurry to acurrent-collector for cathode such as an aluminum sheet or aluminum meshand then drying it. The binder may be chosen from, but not limited to,polyvinylidene chloride (PVdF), polyacrylonitride (PAN), poly vinylchloride (PVC), methyl methacrylate (PMMA), poly methyl acrylate (PMA)or other suitable binders as would be known to a skilled person in theart. The conductive material may be chosen from, but not limited to,carbon black, graphene, or graphite.

The electrolyte may be chosen from, but is not limited to, lithiumhexafluorophosphate (LiPF₆), lithium hexafluorarsenate (LiAsF₆), lithiumperchlorate (LiClO₄), lithium bis(trifluoromethanesulfonimide)(LiN(CF₃SO₂)₂) and lithium trifluorosulfonate (CF₃SO₃Li).

When the porous particles (such as carbon containing sulfur particles)are used in a battery, due to the presence of the electricallyconductive shell, the dissolution and subsequent leakage of lithiumpolysulfides generated during a battery discharge may be substantiallyprevented or reduced. The lithium polysulfides may be generated due tothe reduction of the elemental sulfur on the cathode in the presence ofthe lithium cation (which may be present in the electrolyte as itdissociates from the anode or which may initially be present in theelectrolyte) to form lithium polysulfides (such as Li₂S_(n), where n canbe 8, 6, 4, 3, 2 or 1 depending on the discharge process). Where n is 1,the lithium polysulfide is termed as lithium sulfide. The lithiumpolysulfides then oxidize during charging (which involves thedissociation of the lithium cation from the lithium polysulfides)through a series of intermediary complexes of Li₂S_(n) (n being 2, 3, 4,6, or 8), depending on the charge process, that may result in theformation of the elemental sulfur or a final lithium polysulfide.Furthermore, as the lithiation of sulfur during discharge results in anincreased volume of about 80%, the material encapsulating the sulfur mayallow for the increase in volume that happens when the reaction betweenthe sulfur and the lithium is complete. Lastly, the encapsulatingmaterial may compensate for the poor electrical conductivity of sulfur.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate a disclosed embodiment and servesto explain the principles of the disclosed embodiment. It is to beunderstood, however, that the drawings are designed for purposes ofillustration only, and not as a definition of the limits of theinvention.

FIG. 1 is a schematic diagram representing the method of forming aporous particle in accordance to one embodiment.

FIG. 2A is a scanning electron microscopy (SEM) image of ZnS sphericalnanoparticles obtained via a hydrothermal method in accordance withExample 1. The scale bar in FIG. 2A is 1 micron.

FIG. 2B is a transmission electron microscopy (TEM) image of ZnSspherical nanoparticles obtained via a hydrothermal method in accordancewith Example 1. The scale bar in FIG. 2B is 50 nm.

FIG. 3A is a TEM image of carbon coated ZnS hollow spheres. The scalebar in FIG. 3A is 50 nm.

FIG. 3B is a TEM image of a carbon nanoparticle containing sulfur, inwhich the nanoparticle has a yolk-shell configuration. The scale bar inFIG. 3B is 50 nm.

FIG. 4A is a SEM image of a carbon coated ZnS nanoparticle. The scalebar in FIG. 4A is 1 micron.

FIG. 4B is a TEM image of the same carbon coated ZnS nanoparticle asFIG. 4A. The scale bar in FIG. 4B is 200 nm.

FIG. 4C is a SEM image of a carbon containing sulfur nanoparticledenoted as CS1 made in accordance with Example 1. The scale bar in FIG.4C is 1 micron.

FIG. 4D is a TEM image of the same nanoparticle as FIG. 4C. The scalebar in FIG. 4D is 200 nm.

FIG. 4E is a SEM image of a carbon containing sulfur nanoparticledenoted as CS2 made in accordance with Example 1. The scale bar in FIG.4E is 1 micron.

FIG. 4F is a TEM image of the same nanoparticle as FIG. 4E. The scalebar in FIG. 4F is 200 nm.

FIG. 4G is a SEM image of a carbon containing sulfur nanoparticledenoted as CS3 made in accordance with Example 1. The scale bar in FIG.4G is 1 micron.

FIG. 4H is a TEM image of the same nanoparticle as FIG. 4G. The scalebar in FIG. 4H is 200 nm.

FIG. 4I is a SEM image of a carbon containing sulfur nanoparticledenoted as CS4 made in accordance with Example 1. The scale bar in FIG.4I is 1 micron.

FIG. 4J is a TEM image of the same nanoparticle as FIG. 4I. The scalebar in FIG. 4J is 200 nm.

FIG. 5A is a graph comparing the various X-ray powder diffractions of acommercial sulfur, a carbon-containing sulfur nanoparticle and acarbon-containing ZnS nanoparticle. Graph from top to bottom: CommercialS, S@C and ZnS@C.

FIG. 5B is a graph comparing the thermal gravimetric analysis of thevarious carbon containing sulfur nanoparticles (CS1, CS2, CS3 and CS4)made in accordance with Example 1. Graph: CS1 (—), CS2 (- - -), CS3 (⋅ ⋅⋅) and CS4 (- ⋅ -).

FIG. 6 is a graph showing the Li—S battery performance test of thevarious carbon containing sulfur nanoparticles (CS1, CS2, CS3 and CS4)made in accordance with Example 1. Graph: CS1 (▾), CS2 (▴), CS3 (●), CS4(▪) and coulombic efficiency (solid line).

FIG. 7 is a schematic diagram representing a method as depicted inComparative Example 1 in which a porous carbon nanoparticle wasimpregnated with sulfur using a conventional melt-diffusion process.

FIG. 8 is a SEM image at 20,000× magnification of the resultant productobtained from the method of FIG. 7, showing the presence of micron-sizedsulfur particles.

FIG. 9 is a graph comparing the Li—S battery performance between thecarbon containing sulfur nanoparticle denoted as CS1 made in accordancewith Example 1 and the carbon containing melt impregnated sulfurnanoparticle denoted as MCS made in accordance with Comparative Example1.

FIG. 10A is a elemental mapping of the carbon containing sulfurnanoparticle in which sulfur is present in the core of the particlewhich has a carbon shell.

FIG. 10B is an elemental mapping of the carbon shell portion of thenanoparticle of FIG. 10A.

FIG. 10C is an elemental mapping of the core (sulfur) portion of thenanoparticle of FIG. 10A.

DETAILED DESCRIPTION OF DRAWINGS

Referring to FIG. 1, there is shown a schematic diagram of a method 100of forming a porous particle 2. The porous particle 2 comprises anelectrically conductive shell 20 encapsulating an elemental compound 22in the core. In the method 100, a plurality of elemental compoundprecursor 4 particles are mixed with carbon precursor 6 to form amixture of both precursors 8. The carbon precursor may be in a resinform so as to hold the elemental compound precursor 4 particles in amatrix configuration. The mixture 8 is then subjected to a forming step10 (such as a carbonization step) to form the electrically conductiveshell 20 from the carbon precursor 6. The particle 12 then comprises anelectrically conductive shell 20 encapsulating the elemental compoundprecursor 4. The particle 12 is then subjected to oxidation in thepresence of an oxidation agent 14 to oxidise the elemental compoundprecursor 4 to elemental compound 22. The resultant porous particle 2 isthen formed. When the porous particle 2 is part of a cathode of arechargeable battery, the elemental compound 22 within the porousparticle polymerizes during discharge of the cathode to form apoly(elemental compound) 24 (that is still encapsulated by theelectrically conductive shell 20). The poly(elemental compound) 24dissociates during charging of the cathode to revert back to theelemental compound 22.

EXAMPLES

Non-limiting examples of the invention and a comparative example will befurther described in greater detail by reference to specific Examples,which should not be construed as in any way limiting the scope of theinvention.

Experimental

General Synthetic Procedure of Sulfur Containing Carbon Yolk-ShellNanostructures:

The method 100 depicted in FIG. 1 is used here to synthesize zincsulfide nanoparticles (ZnS NP) by hydrothermal process. In the process,equal mol of zinc acetate dehydrate (with 2 mol H₂O), Zn(CH₃COO)₂·2H₂O,and thiourea, H₂NC(═S)NH₂, were added into deionized water to form theprecursor solution. Gum arabic was then added as a surfactant for theformation of the nanospheres. The mixture was stirred and sonicated toensure complete dissolution of the reagents, before it was transferredinto an autoclave which was then sealed and placed into an oven for areaction at 120° C. over 12 hours. The resultant white precipitate ofzinc sulfide was retrieved via centrifugation, washed with deionizedwater for 3 times and dried at 70° C. for 3 hours. A small volume ofacetone solution of phenol formaldehyde (PF) resin was thoroughly mixedwith the zinc sulfide nanoparticles through stirring and sonication forabout 10 minutes before the mixture was dried in a vacuum over 5 hours.The samples were then subjected to high temperature treatment in a tubefurnace at 900° C. under argon gas for 1 hour. The treated samples wereground into fine powder using a mortar and pestle, and ferric nitrateaqueous solution was added to the sample and left with stirringovernight in an ice water bath to convert the zinc sulfide to sulfur.The sulfur-in-carbon samples were recovered via centrifugation, andconcentrated HCl was added to each sample as a precaution to remove anypossible zinc sulfide residue. After the removal of HCl viacentrifugation, the obtained sample was further washed 3 times withdeionized water, subsequently dried at 70° C. for 3 hours, prior to usefor further characterization and battery testing. All chemicals usedwere obtained from Sigma-Aldrich (of Missouri of the United States ofAmerica) or from Alfa Aesar (of Massachusetts of the United States ofAmerica).

Characterization:

Scanning Electron Microscope (SEM) images were taken on a JEOL JSM-6700FFESEM with an accelerating voltage of 5 kV. Transmission ElectronMicroscope (TEM) images were taken on a Philips CM300 FEGTEM with anaccelerating voltage of 200 kV. X-ray diffraction was recorded on BrukerD8 General Area Detector Diffraction System using Cu Kα radiation.Thermogravimetric analysis was conducted on a TA instruments TGA Q500 ata heating rate of 10° C. min⁻¹ under nitrogen gas. Elemental analysiswas carried out using the Flash EA1112 Elemental Analyzer from ThermoScientific.

Battery Testing:

To fabricate the working cathodes for battery testing, the samples wereeach mixed with carbon black (Denka) and polyvinylidene binder dissolvedin N-methyl-2-pyrolidinone in a weight ratio of (8:4:3) to form a blackcolored slurry. This slurry was then coated evenly onto an aluminum foilusing doctor blade. The foil was dried in an oven at 70° C. for 3 hoursand the working cathodes were cut out from the foil using a hole puncherwith a diameter of 15 mm. CR2032 type coin cells were assembled in aglove box with argon environment using lithium foil as counter anodes.The electrolyte used was lithium bis(tri-fluoromethanesulfonyl)imidedissolved in 1:1 (v/v) mixture of 1,2-dimethoxyethane and 1,3-dioxolane,with lithium nitrate and dilithium hexasulfide (Li₂S₆) as additives. Theconcentrations of the 3 lithium salts were 1M, 0.5M and 0.2M,respectively. Lithium-sulfur battery cycling tests were performed onNeware battery tester with the electric potential set as 1.5 to 3.2 V.Capacity values was calculated based on the weight of compositematerial. The mass loading of composite (carbon and sulfur) is around2.2 mg per electrode.

Example 1—Synthesis of Sulfur Containing Carbon Yolk-ShellNanostructures

ZnS nanoparticles were prepared using 65.85 g (0.3 mol) of zinc acetatedihydrate (FW: 219.50 g/mol) and 22.84 g (0.3 mol) of thiourea (FW:76.12 g/mol) following the method described above. The amount of gumarabic used was 3 g. For the carbon coating formation, 0.5 g of ZnSnanoparticle was taken, and the PF resin acetone solution had aconcentration of 0.5 g mL⁻¹. Four samples of the ZnS nanoparticles wereprepared in this example based on varying amounts of the PF resinacetone solution used (0.5 mL, 0.4 mL, 0.2 mL and 0.1 mL). For thesulfur formation process, the ferric nitrate applied had a concentrationof 0.1 g mL⁻¹, and the volume used was 20 mL. The resultant samples weredenoted as CS1, CS2, CS3 and CS4, respectively

The weight ratio of sulfur and carbon obtained in the nanoparticles asassessed by thermogravimetry is disclosed in the following Table 2:

TABLE 2 TGA Analysis Sample Sulfur (wt. %) Carbon (wt. %) CS1 33.8 66.2CS2 42.9 56.9 CS3 54.0 46.0 CS4 74.8 25.2

The ZnS nanoparticles synthesized were typically about 250 nm indiameter (FIG. 2A and FIG. 2B). From the TEM image (FIG. 2B), one cansee that these uniform particles were essentially secondary particles,arising from the assembly of ZnS nanoparticles (primary particles) witha diameter of 4 to 5 nm. Such structure showed the followingadvantages: 1) it possessed large population of micropores that allowedthe oxidative ferric ions to easily impregnate into the nanoparticles;2) the much smaller primary particles were more reactive; 3) some voidswere reserved in the materials. Advantages (1) and (2) enabled easierand faster conversion of sulfide to sulfur, while advantage (3)facilitated the formation of the S@C yolk-shell structure (see FIG. 3Bwhere the sulphur yolk is circled) with a void having a large volumepercentage (as will be discussed later).

When the ZnS nanoparticles containing phenol formaldehyde (PF) acetonedispersion was slowly dried at room temperature, the nanoparticlesself-assembled into layered structure with PF resin filling theinter-particle voids which essentially coated over all these particles.As the dried composite (ZnS@resin) was carbonized in a tube furnace athigh temperature in an inert atmosphere, interconnected carbon-coatedZnS nanoparticles were obtained. Representative SEM and TEM images ofsuch composite material post-carbonization (ZnS@C) are shown in FIG. 3A,FIG. 4A and FIG. 4B, where carbon-coated ZnS hollow spheres are clearlyvisible.

As the ground ZnS@C composite was soaked in ferric nitrate aqueoussolution, the chemical reaction shown in equation (2) took place wherezinc sulfide was converted to element sulfur and free zinc ions whichcan be washed away using deionized water.[Math. 2]2Fe³⁺ _((aq))+ZnS_((s))→2Fe²⁺ _((aq))+Zn²⁺ _((aq))+S_((s))  (2)

Loss of the zinc ions which contributed to ⅔ of the weight of zincsulfide resulted in the generation of voids of large percentage volumewithin the carbon shells. Though the produced sulfur may exhibit avolume that is larger than its partial volume in the original ZnS, it byno means compensated for the loss in volume caused by the removal ofzinc ions. In an ideal conversion where all sulfides are converted tosulfur that is retained in the shell, a volume reduction of 34.7%(smaller than the weight portion of ⅔) can be anticipated. The voidswill present a larger volume percentage for incomplete conversion of ZnS(removed by washing with concentrated HCl). Hence, the void volume canbe tuned by controlling the extent of sulfide conversion which isachievable by varying reaction conditions, such as the concentration offerric nitrate, the soaking time, etc. In this way, carbon encapsulatedsulfur nanoparticles (S@C) in yolk-shell nanostructure can be harvestedwith controlled void volume percentage.

In this Example, the average volume shrinkage is typically 80%, if theouter diameter of the sulfur nanospheres and the internal diameter ofthe “hollow carbon” are measured in the TEM images. Since volumetricexpansion of sulfur during discharge process in lithium sulfur batteriesis calculated to be about 78.7%, this incidental match betweenvolume-available and volume-in-need suggests a straightforwardproduction of useful S@C materials based on the method defined herein.

The sulfur content in S@C can be balanced by changing the amount of PFresin used, and TEM images of a series of the yolk-shell S@C withdifferent sulfur content are shown in FIG. 4C to FIG. 4J. Low sulfurcontent (FIG. 4C to FIG. 4F) generally facilitated the formation of aninterconnected 3D carbon matrix that is beneficial to electronconductivity, yielding better battery performance at highcharge-discharge rates. With the increase of sulfur content, thethickness of carbon shell will be reduced. Once the sulfur contentreached 50 wt % and above (FIG. 4G to FIG. 4J), the carbon matrix wasbroken and individual S@C particles formed. In this case the electronconductivity was compromised, which worsens the rate performance inbattery test. Nevertheless, high sulfur content is essential for highspecific capacity in batteries. Therefore, it is crucial to balance thecarbon/sulfur ratio so that a continuous carbon matrix with maximizedsulfur content can be achieved. From the TEM images, an optimizedthickness of the carbon coatings is about 5 nm and the diameter of thesulfur nanospheres contained within these carbon shells is about 140 nm.

The structure of ZnS@C and S@C composites were verified by powder XRDpatterns, as shown in FIG. 5A. It can be seen that the ZnS obtained viahydrothermal method possessed a cubic phase (zinc blende or sphalerite),and the three strong peaks at 28.5°, 47.5° and 56.4° corresponded to thecrystal planes of (111), (220) and (311), respectively. For S@C, all thediffraction peaks from the samples matched well with that of theorthorhombic phase of crystalline sulfur (XRD pattern of commercial S isshown as reference), proving the existence of such crystalline sulfur inthe composite material.

The thermogravimetric analysis was used to determine the actual sulfurcontent in the S@C composite obtained, and the profiles are presented inFIG. 5B. The weight loss from 150 to 500° C. was attributed to theevaporation of sulfur (melting point: 115.2° C.; boiling point: 444.6°C.). Details of the TGA profiles were analyzed and the calculated sulfurcontent for all S@C composites was summarized in Table 2 above. It isobvious that by controlling the amount of PF resin used, the sulfurcontent was tunable between 33.8 wt % to 74.8 wt %. The composition ofthe S@C composites was also further confirmed by the data obtained inelemental analysis.

Example 2—Battery Performance

The cycling performance of the developed S@C yolk-shell materials wastested in Li—S batteries and the results are shown in FIG. 6. Theinitial discharge capacity of samples CS1, CS2, CS3 and CS4 were 443,548, 796 and 878 mAh gram per electrode (including composite, carbonblack and binder), respectively. The rate of capacity fading was verysimilar for CS1 to CS3 with thicker carbon shell; however, a much fastercapacity fading was observed for CS4 that possessed the thinnest carbonshell and the highest capacity. After 20 cycles of discharge-chargeprocesses, the discharge capacity of CS4 dropped to a value which islower than that of CS3. The poorer cyclability of CS4 can be attributedto the thin carbon shell that was inadequate to prevent the polysulfidesfrom leaking out of the carbon spheres. Therefore, it is crucial to havea balanced C and S content for high capacity and good cyclability ofLi—S batteries, and the optimal ratio is around 1:1.

Comparative Example 1—Synthesis of Reference Sample (Porous CarbonNanostructures with Melt-Impregnated Sulfur)

Fresh prepared carbon-sulfur composite (for example CS2) was soaked intoluene for 1 hour to remove the sulfur within the pores. The resultingporous material was recovered by centrifugation, washed repeatedly withtoluene and then dried in oven at 70° C. for 12 hours. The obtainedporous carbon was mixed with elemental sulfur in a 1:1 weight ratio andsulfur was impregnated into empty pores via the conventionalmelt-diffusion process in the autoclave at 150° C. for 12 hours. Thefinal product was ground into fine powder using a mortar and pestle, anddenoted as MCS. The method is summarised in FIG. 7.

The sample MCS exhibited poor morphology control. A representative SEMimage (FIG. 8) of MCS showed ill-defined morphology with numerousmicron-sized sulfur particles on the surface of carbon matrix. As theelectrical conductivity of sulfur is very low, these micron-sized sulfurparticles gave poor electrical contact in the battery electrode andprevented the complete utilization of available sulfur, leading to muchlower capacities. This is clearly shown in FIG. 9, where MCS with 50 wt% of sulfur displayed a capacity lower than that of CS1 (33.8 wt % ofsulfur). The higher capacity fading rate for MCS in FIG. 9 was alsocaused by the unprotected sulfur on the surface of the carbon shellsthat readily contributed to the redox shuttle and gradually precipitatedon the anode, leading to faster capacity fading.

This comparative example shows the problems with using a melt-diffusionprocess to impregnate a porous carbon with the sulfur. Together withanother convention process of using vapour-phase infusion, these methodsresult in poor control over the sulfur filling content in individualpores. Either overfill (>>50% (v/v) of the pore) or underfill (<<50%(v/v) of the pore) will have significant negative impact on the batteryperformance. Apart from poor control over the sulfur filling content,some sulfur will unavoidably be deposited on the surface of the hostmaterial. Such unprotected sulfur will contribute readily to the redoxshuttle, resulting in large capacity losses during the initial cycles.

Hence, it is clear that the S@C nanoparticles made according to themethod as defined herein, that is, carbon-encapsulated sulfur inyolk-shell nanostructure from a ZnS precursor approach, is superior tothe MCS sample that was made according to a conventional melt-diffusionprocess in term of the battery performance.

In summary, a new method to prepare S@C nanoparticles with well-definedyolk-shell nanostructure has been developed. The well-defined yolk-shellnanostructure of the S@C nanoparticles can be seen in FIG. 10A (thepresence of the sulfur yolk within the carbon shell), FIG. 10B (showingthe carbon shell) and FIG. 10C (showing the sulfur present as the core).

Such composite nanoparticles offer sufficient free volume to accommodatethe expansion of sulfur during discharge process, and the effectivecarbon coating (which, as discussed above, cannot be too thin) canprevent excessive polysulfide leakage. Cathodes of such S@C materialsexhibited high initial capacities and excellent cycling performance. Thesuperiority of this method over traditional melt-diffusion methods wasthat (1) an even dispersion of sulfur nanoparticles can be achievedinside the pores, and (2) formation of bulk sulfur particles on thesurfaces of the carbon matrix is absent or minimized. Both are crucialto a good cyclability of the lithium sulfur batteries.

INDUSTRIAL APPLICABILITY

The method as defined herein can enable the synthesis of different novelnanostructures for future high performance lithium-sulfur batteries withhigh specific capacity and good cyclablility. Advantageously, due to theuse of the disclosed method, as compared to traditional melt-diffusionmethods, (1) an even dispersion of sulfur nanoparticles can be achievedinside the pores, and (2) formation of bulk sulfur particles on thesurfaces of the carbon matrix is absent or at least substantiallyreduced.

The porous particle made according to the method as defined herein maydisplay superior performance when used as cathode material in arechargeable Li—S battery, such as high initial capacities, goodelectric conductivity, high specific capacities and/or excellent cyclingperformance. In addition, the leakage of sulfur and polysulfides duringbattery discharge can be substantially reduced due to the presence ofthe shell that confines the sulfur within the nanoparticle. Moreover,due to the presence of the void in the nanoparticle, the sulfur canvolumetrically expand during the discharge process while not damagingthe shell. Further, due to the interconnected 3D carbon matrix betweenthe various carbon containing sulfur nanoparticles, this results in goodelectrical contact.

It will be apparent that various other modifications and adaptations ofthe invention will be apparent to the person skilled in the art afterreading the foregoing disclosure without departing from the spirit andscope of the invention and it is intended that all such modificationsand adaptations come within the scope of the appended claims.

What is claimed is:
 1. A porous particle comprising an electricallyconductive shell encapsulating a core, said core comprising an elementalcompound precursor that oxidizes in the presence of an oxidation agentto form an elemental compound, wherein the elemental compound precursoris a metal chalcogenide comprising a metal selected from Group 7, 8, 9,10, 11, 12, or 14 of the Periodic Table of Elements.
 2. The porousparticle according to claim 1, wherein said elemental compoundreversibly reduces in the presence of a cation and oxidizes in theabsence of said cation.
 3. The porous particle according to claim 1,wherein a void is present between said shell and said elementalcompound.
 4. A cathode comprising a plurality of porous particles, eachporous particle made according to a method of forming a porous particlecomprising an electrically conductive shell encapsulating a core, saidcore comprising an elemental compound that reversibly reduces in thepresence of a cation and oxidizes in the absence of said cation, saidmethod comprising: a) encapsulating an elemental compound precursor withsaid electrically conductive shell, wherein the elemental compoundprecursor is a metal chalcogenide comprising a metal selected from Group7, 8, 9, 10, 11, 12, or 14 of the Periodic Table of Elements; and b)reacting said elemental compound precursor with an oxidation agent tooxidize said elemental compound precursor to form said elementalcompound, thereby forming said electrically conductive shellencapsulating said core comprising said elemental compound, wherein eachporous particle comprises an electrically conductive shell encapsulatinga core, said core comprising an elemental compound that reversiblyreduces in the presence of a cation and oxidizes in the absence of saidcation, and wherein a void is present between said shell and saidelemental compound and the volume occupancy of the void between theshell and the core in the oxidized state is between about 10% to about95%.
 5. A battery comprising: a) an anode; b) the cathode of claim 4;and c) an electrolyte comprising said cation.
 6. The battery of claim 5,wherein said cation is dissociated from said anode.
 7. The battery ofclaim 5, wherein said cation is selected from the group consisting oflithium, sodium, potassium, rubidium, beryllium, magnesium, calcium,strontium and barium.